Progressives will be watching some big primary races this Tuesday. But the biggest race of all is in Pennsylvania's Democratic senatorial primary, where groups like MoveOn and Democracy for America have poured resources behind Joe Sestak, backing him against the Obama-backed Sen. Arlen Specter:

Perhaps no race has as much symbolic significance for Obama as the Democratic primary in Pennsylvania between incumbent Sen. Arlen Specter and Rep. Joe Sestak. According to the most recent polls, the two are locked in a virtual tie as they go into Tuesday's voting.

A Specter loss would be viewed by many as a defeat for Obama, even though the president remains highly popular among Pennsylvania's Democrats. That's because Obama was personally involved in wooing Specter to the Democratic Party and promised support in his bid to stay in the Senate. One of the final ads Specter is running features the senator visiting Obama.

The White House signaled more than a week ago that the president would not make another campaign trip for Specter in the final days of the primary race, perhaps wanting to avoid a repeat of the presidential-visit-followed-by-loss sequence that occurred in January when Republican Scott Brown won the open Senate seat in Massachusetts just days after Obama campaigned there for Democrat Martha Coakley.

But a win by Sestak may not offer any lasting damage for Obama. He campaigned as a supporter of the president's, and the congressman could turn out to be a better candidate in the fall against the Republican nominee.

If Specter loses -- or even if he wins by a tiny margin -- it could foreshadow difficulties for all incumbents, as further evidence that voters are fed up with those in power. It would also be an indication of the difficult prospects for party-switchers; Obama is asking many Democrats in Pennsylvania to support Specter after they had spent years campaigning and voting against him.

Democrats have been voting for Specter for a long time, so I don't think that's a real problem. If anything, people will vote against him because they think it's time for new blood.